A nuclear reactor core arrangement in common use comprises a multiplicity of elongated, cylindrical, parallel, fuel-containing rods spaced apart in an upstanding, rectangular array. A liquid coolant, generally having moderating properties, flows vertically along the length of the rods through the interstitial space between the rods. Light and heavy water are widely used liquid coolants which also have moderating properties. Such core arrangement is typically found in, but not limited to, nuclear reactors of the classes designated as pressurized water reactors and boiling water reactors. Similar core arrangements may be found in reactors using other coolants such as liquid metals as used in fast breeder reactors. In some cases, groups of fuel rods are joined together in boxes or cans, which provide mechanical strength, to form fuel subassemblies which are in turn assembled in an upstanding array to form a core. Where no boxes or cans are used, the resulting core is referred to as an open lattice design.
An important aspect of mechanical design associated with the aforementioned core arrangement is the provision therein of support grids for holding the fuel rods in preselected positions. The function of the support grids is to maintain a minimum spacing between fuel rods so that the entire length of all fuel rods can be adequately cooled. Failure to maintain such spacing can result in "hot spots" in the fuel rods with cladding burnout and loss of fission products from the nuclear fuel. Such grids are normally designed to engage the fuel rods directly or end extensions provided to accommodate fission product gases given off by the nuclear fuel within the fuel rods. Consideration must be given to the mechanical strength of the grid, its poisoning effects on the nuclear properties of the core due to neutron absorption in the grid material, and its effect on coolant flow passing along the fuel rods.
Ideally, a support grid should be strong to resist lateral movement by the fuel rods, fabricated of a small amount of material to minimize its poisoning effect, and have a small physical cross sectional area normal to the coolant flow to minimize flow resistance to the coolant as it passes along the fuel rods. Fabrication costs should also be minimized through the use of a grid design which is simple and easily assembled.
A typical grid structure used to support and space fuel rods comprises a sheet metal assemblage formed of intersecting sheet metal strips. To restrain the fuel while allowing for thermal expansion, the fuel rods are held in the grid structure by springs formed in the sheet metal strips. Springs formed integrally in the sheet metal of the grid provide a grid structure characterized by good core neutron economy, structural redundancy, mechanical integrity, and manufacturing variability. The principle disadvantage of typical integral grid springs is the relatively stiff spring characteristic obtained from sheet metal stamping. It is desirable to use a doubly supported beam configuration for redundancy in the event of the spring fracture to prevent loss of the spring into the coolant channels where it could cause flow blockage.
It is, accordingly, a general object of the invention to provide a grid structure formed of sheet metal strips with grid springs for retaining fuel rods formed integrally in the sheet metal strips and grid springs having an initial force value adequate to restrain fuel rod motion under hydrodynamic excitation and a lesser force value thereafter to accommodate large deflections required by manufacturing tolerances and thermal expansion effects.
Other objects of the invention will be apparent from an examination of the following written description and the appended drawings.